1. Field of the Invention
The present invention relates to a copolymer suitable for photoresist. More particularly, the present invention relates to a vinyl 4-hydroxybenzal-vinyl alcohol-vinyl acetate copolymer, a vinyl 4-t-butoxycarbonyloxybenzal-vinyl alcohol-vinyl acetate copolymer and a vinyl 4-t-butoxycarbonyloxybenzal-vinyl 4-hydroxybenzal-vinyl alcohol-vinyl acetate copolymer. In the last two copolymers, all or part of the vinyl 4-hydroxybenzal groups are protected with t-butoxycarbonyl group. Also, the present invention is concerned with methods for preparing the copolymers.
2. Description of the Prior Art
A significant advance has been made for the photoresist used in microlithography, which is one of the most essential techniques to achieve the high integration of semiconductor integrated circuits. As higher integration degree of a semiconductor integrated circuit has been required, various photoresist materials are now developed and reported. Among them, polyvinylphenol resins protected by t-butoxycarbonyl group and poly(4-t-butoxycarbonyloxystyrene) resins are expected to be the most useful and are being used in practice. These resins are very advantageous in that they are directly applied for such techniques advanced in microlithography as utilizes deep uv as a light source for exposure, as disclosed in U.S. Pat. Nos. 4,491,628, 4,405,708 and 4,670,507.
However, the polyvinylphenol resins show high optical absorbance at around 250 nm due to the benzene rings contained. Because deep uv, a light source for the microlithography in current use, has a frequency of around 250 nm, the polyvinylphenol resins are low in transparency at the frequency.
Poly(4-t-butoxycarbonyloxystyrene) resins lose too much weight upon thermal treatment after exposure in addition to being poor in adhesiveness. Like polyvinylphenol resins, they show low transparency because of high optical absorbance at around 250 nm. For example, a resin obtained by using 4-t-butoxycarbonyloxy styrene with an average molecular weight of 10,000 as a monomer and 2,2'-azobisisobutyronitrile (AIBN) as an initiator, shows a uv optical absorbance of 0.170 at 250 nm when it is 1 .mu.m thick. For a resin 1 .mu.m thick polymerized from the monomer in the presence of benzoylperoxide, its uv optical absorbance is 0.140. As exemplified above, both resins are opaque at around 250 nm because of high optical absorbance.